In the face of growing fresh water scarcity concerns worldwide, the perception of water resources is rapidly shifting toward one of more direct cyclic systems of reuse. The United States leads the world in volume of wastewater reused (7.6 million m3/d as of 2008), and more arid countries are obtaining a continually higher fraction of their drinking water from desalination and reclaimed sources. These trends are expected to continue as the supply of fresh water continues to decrease due to increasing municipal and agricultural demands as well as due to modern oil and gas extraction practices, which are generating increasing volumes of wastewater. A single hydraulic fracturing well, for example, can use over 3 billion gallons of water per year, much of which emerges as brine waste.
The practice of wastewater and drinking water treatment are thus converging into a single issue of resource management, creating new environmental engineering challenges. Advancement of reverse osmosis, nanofiltration, and advanced oxidation process technologies will directly impact the cost and availability of drinking water in the 21st century, with the potential to offer powerful treatment capabilities with modest space requirements.
Membrane filtration (e.g., nanofiltration) is widely used in industrial applications due to its ability to efficiently remove virtually particles larger than about 0.2 μm, including bacteria such as Giardia lamblia and Cryptosporidium parvum. Membranes are also a critical component in reverse osmosis desalination plants. As such, the use of membrane technologies has greatly increased over the course of the last two decades. As an example, the global installed capacity for low-pressure membrane systems, including drinking water, wastewater, and industrial water treatment plants, has grown from approximately 100 million gallons per day (MGD) in 1996 to almost 3,500 MGD in 2006.
Although membrane-based water treatment is an established industry, existing membrane technology is far from providing optimal sustainability, primarily due to performance decline caused by compaction, fouling, repeated cleaning to alleviate fouling, and resulting gradual deterioration of the membrane material. Biofouling in particular is one of the greatest operational challenges associated with reverse osmosis and nanofiltration, occurring when excessive biofilm accumulation within feed channels and on membrane surfaces degrades performance Bacterial biofouling is frequently encountered in systems in which source waters include brackish surface waters, e.g., rivers and coastal areas, which frequently include relatively large populations of bacteria. Bacteria easily colonize membrane surfaces in the treatment system environment and can form a thick biofilm mat that is difficult to eradicate once established.
Biofouling is not limited to bacterial sources, however, and other sources of biofouling include algae and fungi; bioproducts of any of these living organisms, such as humic acid or other organics; and combinations thereof. Any of these biofouling sources may clog a membrane and reduce flow, and may provide nucleation sites for scale deposits, which also inhibit flow. In either case, the result is reduced membrane performance as well as, frequently, degradation of the membrane polymer itself.
The primary method utilized to deal with biofouling has been to replace membranes once they have become unacceptably fouled. This, however, disrupts operations and is economically undesirable, leading to slower acceptance of membrane-based environmentally friendly treatment options for large water treatment facilities. Another method is to treat the membranes off-line to remove the biofouling. This also disrupts operation and is relatively costly. Still another approach is the in-line use of biocides such as DBNPA (2,2-bromo-3-nitrilopropionamide). Such compounds can be very effective at killing biofouling sources, but utilization has been limited to production of water for industrial purposes due to the concern that biocide levels effective to treat the membrane could contaminate the permeate water with the biocide or its by-products and render the permeate unacceptable for municipal water use. Because of this concern, the water produced during treatment with these biocides is, at present, discarded as waste.
Halogens, primarily chlorine in the form of sodium hypochlorite or chlorine gas, have been used to control biofouling. However, this treatment option requires a subsequent dehalogenation step in order to prevent the halogen from actually contacting and degrading the membrane surface and/or passing through to the permeate side of the membrane. The additional dehalogenation step adds to the expense and inconvenience of the water production process. Variations of the halogen approach have included combining an oxidizing biocide containing a halogen with a nitrogen compound, which helps to bind the halogen and thereby reduce its contact with the membrane. Examples of these combination oxidizing biocide materials include bromochlorodimethylhydantoin (BCDMH) and trichloro-isocyanuric acid. Other approaches to membrane biofouling have included use of peracetic acid, ultraviolet light, and ozone pretreatment operations. Unfortunately, peracetic acid will often accelerate the degradation of the membrane, and the ultraviolet light and ozone methods suggested to date have been extremely cost-intensive, particular when considered for use on the scale necessary for municipal water production.
What are needed in the art are methods and systems that can prevent biofouling of water treatment membranes without disrupting operation. Methods that can extend the life of treatment membranes in a cost-effective manner would be of great benefit.